1. The Field of the Invention
The present invention relates generally to optoelectronic components. More particularly, the present invention relates to systems and methods for maintaining a CWDM transmitter emitting within a target wavelength channel over an extended temperature range.
2. The Relevant Technology
Computer and data communications networks continue to develop and expand due to declining costs, improved performance of computer and networking a equipment, the remarkable growth of the internet, and the resulting increased demand for communication bandwidth. Such increased demand is occurring both within and between metropolitan areas as well as within communications networks. Moreover, as organizations have recognized the economic benefits of using communications networks, network applications such as electronic mail, voice and data transfer, host access, and shared and distributed databases are increasingly used as a means to increase user productivity. This increased demand, together with the growing number of distributed computing resources, has resulted in a rapid expansion of the number of fiber optic systems required.
Through fiber optics, digital data in the form of light signals is formed by lasers or light emitting diodes and then propagated through a fiber optic cable. Such light signals allow for high data transmission rates and high bandwidth capabilities. The light signals are then received at a photodiode which converts the light signal back into an electrical signal. Current optical designs typically include both the laser and the photodiode within a single transceiver module that can be connected to a host device, such as a host computer, switching hub, network router, switch box, computer I/O and the like, via a compatible connection port at one end and to a fiber optic cable at the other. Each transceiver module typically contains, in addition to the laser and photodiode, all the other optical and electrical components necessary to change electrical signals to optical signals via the laser and optical signals back into electrical signals received at the photodiode.
Another advantage in using light as a transmission medium is that multiple wavelength components of light can be transmitted through a single communication path such as an optical fiber. This process is commonly referred to as wavelength division multiplexing (WDM), where the bandwidth of the communication medium is increased by the number of independent wavelength channels used. Several wavelengths channels can be transmitted using coarse wavelength-division multiplexing (CWDM) applications to achieve higher channel density and total channel number in a single communication line.
FIG. 1 illustrates eight wavelength channels that are typically used in CWDM systems. As illustrated, CWDM typically implements a channel spacing of 20 nanometers. Thus, CWDM allows a modest number of channels, typically eight or less, to be stacked around the 1550 nm region of the fiber. FIG. 1 illustrates how CWDM transmission may occur at one of eight wavelengths: typically 1470 nm, 1490 nm, 1510 nm, 1530 nm, 1550 nm, 1570 nm, 1590 nm, and 1610 nm.
In order to save cost and reduce power consumption, CWDM transmitters conventionally use uncooled lasers with a relaxed, room temperature tolerance of ±3 nm. The wide spacing accommodates the uncooled laser wavelength drifts that occur as the ambient temperature varies within a relatively small acceptable range.
FIG. 2 illustrates in greater detail three adjacent channels in a CWDM system. Each channel uses a filter that has a pass-band approximately 11 nm wide. Operation outside the allowable allowed pass-band results in high attenuation of the transmitted signal, and in extreme cases, potential cross-talk with the adjacent channel. It is not necessary that the emitted light from a transmitter occupy the entire channel, only that it stays within it. Thus, it can be seen that the spectrum of the light transmitted from a transmitter may occupy the wavelength range illustrated by wavelength range 12 in the center of the passband. It is also acceptable if the transmitted light is on one side of the wavelength channel, for example wavelength range 14. A CWDM system does not operate properly, however, if a transmitter emits light in a wavelength range that is outside the designated passband or overlaps with an adjacent passband, by way of example wavelength range 16.
There are several factors determining the wavelength of a signal produced by traditional laser sources. These factors include, for example, current density, temperature of the light emitter, and the specific inherent characteristics of the light emitter. With reference to FIG. 3, a chart depicting wavelength (λ) shift over a varying temperature range is depicted. The range of wavelengths is depicted along the y axis, having no particular wavelengths indicated and the range of temperatures is depicted along the x axis, from −40° C. to 85° C. A line 20 is shown to depict the characteristic wavelength shift as temperature increases. It is generally accepted that a wavelength drift of 0.1 nm per degree Celsius shift if a good approximation of the shift for Distributed Feedback (“DFB”) laser sources that are commonly used in CWDM applications. It can therefore be seen how the wavelength of the emitted light shifts approximately +12.5 nm over the 125 degree temperature change. This is illustrated by the slope of plot 20, with the low point 22 of the slope at −40° C. being 12.5 nm below the high point 24 at 85° C.
In order to control the effects of temperature-caused wavelength drifting, CWDM transceivers are typically cooled by devices external to the transceiver, such as fans and their placement in controlled temperature rooms. The controlled environment maintains the transceiver component within a reasonable temperature range so that the laser emits at a wavelength range within the designated wavelength channel.
Using CWDM transceivers only in controlled environments can be very restrictive and expensive, however. As a result, there has arisen an interest to operate CWDM transceivers in less expensive or more convenient locations. For example, it would represent and advance in CWDM technology if a CWDM transceiver could be operated in the field, for example at a data relay station in a remote location. In fact, the desired operating conditions for CWDM transceivers are currently as large as from −40° C. to 85° C.
As a result, what are needed are devices and methods to enable CWDM transceivers to be operated in an environment having large temperature variations without experiencing excessive wavelength drift. In particular, it would represent an advance in the art to enable the operation of CWDM transceivers without the use of fans or their placement in controlled temperature rooms.